Sarcopenia

Sarcopenia, from a Greek word meaning ‘‘poverty of flesh,’’ is the loss of muscle mass and strength caused by normal aging. It is distinct from muscle loss caused by inflammatory disease (cachexia), and from the weight loss and attendant muscle wasting caused by starvation or advanced disease. Compared to young, healthy, physically active young adults, reduced muscle mass and strength are evident in all elderly persons. If the sarcopenia progresses beyond a threshold of functional requirements, it leads to disability and frailty, and this can occur independently of any disease-induced frailty. Of course, a superimposed illness will accelerate the loss of muscle mass, and thus increase the risk of disability, frailty, and death.

There is no absolute level of lean mass, body cell mass, or muscle mass at which one can definitely say that sarcopenia is present. However, it is important to consider two important and generally agreed-upon concepts in relation to lean body mass. First, there is a direct structure-function link between muscle mass and strength—more muscle generally equals greater strength, and vice versa. Second, there is reasonable evidence that there is a limit on how much lean body mass can be lost before death supervenes. The available data, based on patients suffering from starvation, AIDS, and critical illnesses, suggest that loss of more than about 40 percent of baseline lean mass is fatal. Kehayias, et al. (1997) defined baseline lean mass as the mean for adults between twenty and thirty years of age; no healthy elderly adults were found below approximately 70 percent of that standard, and there was a steady decline in body cell mass for both men and women across age groups between the ages of thirty and one hundred.

This decline in body cell mass with age raises the issue of the importance of sarcopenia as an indicator of reduced protein stores for times of stress. During illness, protein is burned for energy in excess of the levels seen in starvation adaptation. Given the anorexia caused by acute illness, endogenous protein stores are crucial in determining the availability of metabolic substrate needed to cope with the illness, and thus the ability to survive it. It is no wonder, then, that elderly, sarcopenic patients fare worse than young, healthy adults for almost all diseases. For this reason, the metabolic significance of sarcopenia in illness should be considered independently of its functional impact during times of better health, as both are important to the survival and well-being of elderly persons.